Conventionally, the RFID (Radio Frequency Identification) system, which is configured of a unit holding an inherent ID and a unit reading it, has a problem that erroneous detection and an omission of detection occur, and hence, the detection of the ID becomes unstable due to influences such as an antenna directivity of the ID holding unit and the ID reading unit, a disorder in a radio wave environment caused by a multi-path etc., a change in an antenna characteristic caused by interference due to a human body and other non-human objects to which the ID holding unit is attached, and a radio wave noise from a personal computer arranged in the circumference.
So as to decrease the erroneous detection and the omission of detection in the RFID system and to stabilize the detection, in Patent document 1, positively changing a positional relation between the ID holding unit and the ID reading unit, and simultaneously therewith, causing a plurality of the ID holding units not to positionally overlap with each other allow the situation such that a radio wave necessary for reading can be obtained and created temporarily.
Further, in Patent document 2, arranging one or a plurality of block plates for putting restrictions to an area in which the ID reading unit makes the detection allows an arrival area of the radio wave to be restricted.
In Patent document 3 and Patent document 4, housing the ID holding unit into an outer shell in a manner that the ID holding unit can be maintained in a state of being always directed to a direction suitable for reading allows the detection to be stabilized.
Further, in Patent document 5, Patent document 6, Patent document 7, Patent document 8, etc., making an improvement to the unit itself of the RFID system, for example, controlling the transmission power, managing the operating power, designing a circuit for enhancing a signal-to-noise ratio, contriving a transmission/reception antenna structure, allows the detection to be stabilized.
Further, as a general process for aiming at stabilizing the detection, there exist the time-out process in which it is determined that the object of detection has gone out of the detection area in a case where the reading thereof over a constant time was impossible with respect to a history of the read result, the average value computing process of obtaining an average radio wave intensity over a constant time or a read ratio over a constant time with respect to a history of the read result, and the threshold process in which it is determined that the object of detection has gone out of the detection area in a case where the radio wave intensity or the read ratio over a constant time has fallen below a constant value.    Patent document 1: JP-P2002-329174A    Patent document 2: JP-P2004-059320A    Patent document 3: JP-P2003-058843A    Patent document 4: JP-P2002-259932A    Patent document 5: JP-P2002-170082A    Patent document 6: JP-P2001-307052A    Patent document 7: JP-P1999-282977A    Patent document 8: JP-P1998-242893A